In conventional sanitation systems using a septic tank, waste water empties into the septic tank, solids accumulate and decompose on the bottom of the tank and an effluent exits the top of the tank into a "drain field." The drain field, located a few feet below ground level, is intended to disperse the effluent into the soil, where it is filtered. The typical drain field comprises downward oriented conduits with holes at the bottom facing a stone bed, the stone providing a flow path to the surrounding soil from the holes and preventing the soil from filing the holes.
But drain fields can become contaminated over time. Even new drain fields can exhibit the problems of contaminated fields if the water table is too high or the soil is too compacted or otherwise has poor filtering qualities. The basic problem when a drain field is contaminated is that the effluent is not absorbed and backs up to the septic tank. The field is effectively clogged, and that can happen for many reasons: soil compaction; a high water table; bacterial growth in the soil; improper drain inclination; and pipe clogging. When a drain field is clogged, the repair is complicated and expensive. The entire field must be dug up, causing considerable land damage. But repair assumes that there is adequate land to install a new drain field. Smaller plots can make it extremely difficult to lay out drain fields with the separation mandated by local ordinances. Topographical changes can make it impossible to install an effective new drain field. It is common to find that septic systems are used only where building plots are relatively large. The alternative, sewer systems, are expensive to install, and, generally speaking, do not recover water in the effluent.
A typical home can consume a considerable amount of water each day, all of it ultimately entering the drain field where, ideally, it eventually enters the water table, far below, filtered by the soil in the process. In reality, that filtration process's efficiency is limited by soil characteristics and the height of the water table. Also, some water evaporates, especially in hot climates.